The polymerisation of olefin monomers in the presence of catalysts in fluidised bed reactors is well-known. In the fluidised bed polymerisation of olefins the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous olefin. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the olefin, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the had when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid.
The ascending gaseous stream, having passed through the reactor, is withdrawn from the top of the reactor. Given that only a portion of the olefin or olefins reacts when passing through the reactor, the process comprises a recycling loop by which the ascending gas stream withdrawn at the top part of the reactor is recycled to the base. Further, since the polymerisation reaction is highly exothermic the recycling stream is generally cooled to remove the heat of reaction. Current commercial operation generally prefers that the recycle stream is cooled such that a portion thereof condenses to form liquid, and this liquid is also reintroduced into the reactor, either below the fluidisation grid with recycled gas or directly into the reactor, the vaporisation of the liquid in the bed consuming significant quantities of the heat of reaction. Such operation is generally termed “condensed mode” operation.
The polymer particles form by reaction of monomer to form polymer on the catalyst particles, which thus increase with size during reaction. Thus, in a continuous process catalyst is withdrawn with the formed polymer particles, and fresh catalyst must be added to replace it. The particles in the fluidised bed can therefore include a range of particles ranging from relatively small particles which are based on catalyst which has not yet had time to grow to form larger particles on which more polymer has formed. It is also the case that relatively small particles can be present which have been formed by fragmentation of larger particles.
In order to minimise the entrainment of particles out of the reactor, the upper part of the reactor, generally a zone above the fluidised bed in operation, typically comprises a section of expanded cross-section, commonly known a disengagement chamber, the expansion of which causes a reduction in the velocity of the ascending gas stream above the fluidised bed and which allows entrained particles to fall back into the fluidised bed.
Despite the presence of the disengagement chamber significant quantities of smaller particles are nevertheless withdrawn from the top of the reactor in the gas stream.
These particles can be deposited and foul the reactor loop, for example in heat exchangers, compressors and at the fluidisation grid. The fouling is a result of continuing polymerisation under non-favourable conditions (particles at rest with poor heat exchange), and manifests itself most severely as increases in pressure drop through heat exchangers and other components, and reductions in heat transfer across the walls of the heat exchangers.
This fouling can be very severe and can result in it being necessary to shut-down the reactor very regularly (every few months) to clean heat exchangers or other process equipment which has been exposed to the fine particles.
In order to mitigate the problem of fouling, it is also known for the recycling loop to comprise at least one gas/solid separator, preferably a cyclone, capable of separating, horn the gas stream, a substantial portion of the solid particles entrained within it. The solids can then be returned to the reactor, and the rest of the gas stream cooled and recycled as already described.
Despite the presence of the disengagement chamber and a gas/solid separator the gas stream circulating in the loop will still generally contain some solid particles, often referred to as “fines”, and fouling generally still occurs, albeit over a longer timescale than without a cyclone or other gas/solid separator.
A number of further solutions have been proposed to the problem or fouling, especially of heat exchangers.
For example FR-A-2,634,212 provides for injecting a liquid hydrocarbon upstream of a heat exchanger, in order to wash and clean the interior of the exchanger.
Furthermore, WO 98/20046 provides for the introduction, at one or more points in the loop, of an agent which prevents the deposition of polymer particles, and US 2006/094837 seeks to prevent fouling by careful control of the surface temperature of the heat exchanger relative to the dew point of the gas stream, effectively “wetting” the surface.
WO 00/61278 relates to a heat exchanger of a particular design which seeks to prevent the deposition of the fine particles by design of the flow of the gas stream.
Despite all of the above it has generally been the case that it still periodically becomes necessary to shut-down the polymerisation process to clean heat exchangers or other process equipment which has been exposed to the fine particles.